Normal B cell development depends on the proper assembly of immunoglobulin (Ig) heavy and light chain genes from their component gene segments by the V(D)J recombinase. Activity of the recombinase depends on the regulated expression of two lymphoid-specific genes, RAG1 and RAG2 and is regulated such that an individual mature B cell expresses a single antigen receptor specificity. This phenomenon, critical for the regulation of the humoral immune response, is known as allelic exclusion. Work from many laboratories, including our own, has uncovered a role for the assembled immunoglobulin chains themselves in regulating the recombinase. In pre-B cells, heavy-chain mu protein assembles with surrogate light-chains and the receptor accessory chains Ig-a and Ig-b to form the pre-B cell receptor. This complex is involved in inhibiting further heavy-chain gene rearrangement and activating kappa light chain gene rearrangement. In this competing renewal application, we propose a series of experiments aimed at understanding how developing B cells regulate the V(D)J recombinase in response to Ig protein expression. Specifically we plan to 1) test the hypothesis that the stochastic and infrequent activation of enhancers regulating the unrearranged Ig k locus contributes to light chain allelic exclusion; 2) identify critical DNA sequences and transcription factors involved in the regulation of RAG1 and RAG2 transcription; 3) determine whether the CH2 and CH3 domains of Ig mu chain are required of activity of the pre-B cell receptor; and 4) identify proteins capable of binding a region of the Ig k 3' enhancer which may be responsible for the lineage specificity and timing of V-to-Jk rearrangement. Abnormal B cell development is associated with immunodeficiency, autoimmune disease, and malignancy. Failure of the pre-B cell receptor to signal results in developmental arrest and decreased B cell production. Failure to delete or energize self-specific B cells can result in autoimmunity. Finally, many lymphoid malignancies are associated with chromosomal translocations involving the aberrant rearrangement of immunoglobulin gene segments and proto-oncogenes. We perform this research in the hope that understanding normal B cell development will provides basic insights into the etiology these diseases leading to their prevention or effective treatment.